9 CHAPTER 2 LITERATURE SURVEY 2.1 INTRODUCTION This chapter presents a comprehensive literature survey related to the topic of research which includes fabrication of MMCs using different methods with a specific emphasis on in-situ method, dry sliding wear and corrosion behavior of in- situ MMCs and friction stir welding of MMCs. This chapter also includes the Design of Experiments (DOE) technique and its application to different processes. 2.2 FABRICATION OF MMCs The MMCs reinforced with ceramic particles are currently fabricated using different established methods and some specific patented methods. The principles of fabricating the MMCs using traditional methods are briefed in this section. The traditional methods are powder metallurgy, mechanical alloying, stir casting, squeeze casting, compo casting and spray deposition. The processing method influences the mechanical behavior of the MMCs (Kennedy and Wyatt 2000). The successful incorporation of ceramic particles into the matrix alloy and achieving good bonding between them will help to enhance the properties. All processing methods are grouped into two categories which are namely solid state processing and liquid state processing. This grouping is based on the processing temperature which is above (liquid state) or below (solid state) the melting point of the matrix material. The processing temperature of all processes is well below the melting point of ceramic particles. Each process has a limitation to produce MMCs with certain combinations of matrix alloy and ceramic particles. Therefore lot of research emphasis is given to develop the processing methods to fabricate new kind of MMCs whose behavior may be superior to the existing MMCs.
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9
CHAPTER 2
LITERATURE SURVEY
2.1 INTRODUCTION
This chapter presents a comprehensive literature survey related to the topic
of research which includes fabrication of MMCs using different methods with a
specific emphasis on in-situ method, dry sliding wear and corrosion behavior of in-
situ MMCs and friction stir welding of MMCs. This chapter also includes the
Design of Experiments (DOE) technique and its application to different processes.
2.2 FABRICATION OF MMCs
The MMCs reinforced with ceramic particles are currently fabricated using
different established methods and some specific patented methods. The principles of
fabricating the MMCs using traditional methods are briefed in this section. The
traditional methods are powder metallurgy, mechanical alloying, stir casting,
squeeze casting, compo casting and spray deposition. The processing method
influences the mechanical behavior of the MMCs (Kennedy and Wyatt 2000). The
successful incorporation of ceramic particles into the matrix alloy and achieving
good bonding between them will help to enhance the properties. All processing
methods are grouped into two categories which are namely solid state processing
and liquid state processing. This grouping is based on the processing temperature
which is above (liquid state) or below (solid state) the melting point of the matrix
material. The processing temperature of all processes is well below the melting
point of ceramic particles. Each process has a limitation to produce MMCs with
certain combinations of matrix alloy and ceramic particles. Therefore lot of research
emphasis is given to develop the processing methods to fabricate new kind of
MMCs whose behavior may be superior to the existing MMCs.
10
2.2.1 Powder Metallurgy
Powder blending and consolidation is a commonly used method for the
preparation of discontinuously reinforced MMCs. Figure 2.1 shows the typical
processing steps of powder metallurgy composites which are explained as follows.
Powders of the metallic matrix and reinforcement are first blended and fed into a
mold of the desired shape. Blending can be carried out dry or in liquid suspension.
Pressure is then applied to further compact the powder (cold pressing). The compact
is then heated to a temperature which is below the melting point but high enough to
develop significant solid state diffusion (sintering). After blending, the mixture can
also be consolidated directly by hot pressing or hot isostatic pressing to obtain high
density. The consolidated composite is then available for secondary processing such
as extrusion and rolling. Achieving a homogeneous mixture during blending is a
critical factor because the discontinuous reinforcement tends to persist as
agglomerates with interstitial spaces too small for penetration of matrix particles.
Figure 2.1 Processing Steps of Powder Metallurgy Composites (Harrigan 1998)
11
The powder metallurgy processing technique is attractive for several reasons.
This approach offers microstructural control of the phases that is absent from the
liquid phase route. Powder metallurgy processing employs lower temperatures and
therefore, theoretically offers better control of interface kinetics. Several
combinations of matrix alloy and ceramic particles can be used to fabricate MMCs.
Fogagnolo et al (2004) fabricated AA6061/15wt.% ZrB2 MMC using powder
metallurgy technique and achieved uniform distribution of ZrB2 particles in the
matrix. Rahimian et al (2009) developed Al/Al2O3 MMC using powder metallurgy
technique and studied the effect of particle size, sintering time and sintering
temperature on the microstructure and mechanical properties of the MMC.
2.2.2 Mechanical Alloying
Mechanical alloying is a simple and useful technique to synthesize both
equilibrium and non equilibrium phases of commercially useful materials starting
from elemental powders. This method was developed during the late 1960s to
produce high temperature materials. Mechanical alloying produces a homogeneous
distribution of inert, fine particles within the matrix and avoids many problems
associated with melting and solidification.
Figure 2.2 shows a typical mechanical alloying setup used to fabricate
MMCs. The principle of mechanical alloying and operation of the set up are
concurrently detailed as follows. Raw materials used for mechanical alloying are
pure or alloy powders that have particle size ranging from 1 to 200 µm. A process
control agent is added to the powder mixture during milling, especially when the
powder mixture involves a substantial fraction of a ductile component. The process
control agents are mostly organic compounds which act as surface active agents.
The process control agents minimize cold welding and inhibit agglomeration.
Common process control agents are stearic acid, hexane and oxalic acid which are
used at levels of 1–4 wt.% of the total powder charge.
12
Figure 2.2 Mechanical alloying Setup (Arik 2004)
The actual process of mechanical alloying starts with mixing the powders in
the right proportion and loading the powder into the mill along with the grinding
media (generally steel balls). This mix is then milled for the desired time until a
steady state is reached. During high energy milling, the powder particles are
repeatedly flattened, welded, fractured and re-welded. In the early stages of milling,
the particles are soft and their tendency to weld together is high. A broad range of
particle size develops with some particles as large as three times bigger as that of
the starting particles. The composite particles at this stage have a characteristic
layered structure consisting of various combinations of starting constituents. As
deformation continues the particles become work hardened and fractured by a
fatigue failure mechanism. Fragments generated by this mechanism can continue to
reduce in size in the absence of strong agglomeration forces. The final milled
composite powders are then compacted and sintered at high temperatures for certain
duration under argon gas. The consolidated composite is then available for
secondary processing. Mechanical alloyed MMCs have better properties especially
13
at higher temperature due to the reduction of grain size, the high level of work
hardening and the fine dispersion of precipitates in the microstructure (Navas et al
2006).
Fogagnolo et al (2004) used mechanical alloying to fabricate
AA6061/15wt.% ZrB2 MMC and analyzed the effect of milling time on
microstructural evaluation of the MMC. Arik (2004) fabricated Al/Al4C3 MMC
using mechanical alloying and studied the effect of milling time and sintering
temperature on microstructural evolution of the MMC. Navas et al (2006) produced
AA2014/5vol.% TiC MMC using mechanical alloying and the influence of
mechanical alloying parameters on morphology, particle size, microhardness, and
microstructure of the MMC was studied. Zebarjad and Sajjadi (2006) developed
Al/Al2O3 MMC using mechanical alloying and observed uniform distribution of
alumina powders as milling time is increased.
2.2.3 Stir Casting
Stir casting is widely used in industries for mass production of the MMCs.
Figure 2.3 shows a typical stir casting setup which consists of a furnace, crucible
and stirrer. The operation of the setup to produce a MMC is described as follows.
The matrix material (usually kept inside a crucible) is melted in a furnace. The
molten material is stirred to form a vortex. An inert gas is passed to prevent the
formation of oxides. The ceramic particles are fed at a predetermined rate to the
periphery of the vortex. The stirring is continued till all the particles are added. The
composite melt is then poured into die. The solidified composite can be subjected to
heat treatment or rolling to improve the properties.
Selection of process parameters such as stirring speed, stirring time,
temperature of the melt and particle feeding rate are vital to produce quality
composites (Kalaiselvan et al 2011). Improper selection of those parameters will
lead to agglomeration of ceramic particles and high porosity. Wettability is a
significant problem in stir casting. Wettability can be defined as the ability of a
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liquid to spread on a solid surface (Hashim et al 1999). All the ceramic particles are
not wet by molten metals which limit the application of stir casting method to
fabricate MMCs reinforced with Al2O3 and SiC particles for more than three
decades. Al2O3 and SiC are readily wet by the molten aluminum if small amount of
magnesium is added. Efforts are put by researchers to find out suitable wetting
agents and coating materials for other ceramic particles. Compared to other
processing methods stir casting is the most economical.
Figure 2.3 Stir Casting Setup (Kok 2005)
Sahin (2003) fabricated AA2014/20vol.% SiC MMC using stir casting and
examined the tool wear during machining of the MMC. Kok (2005) developed
AA2024/0-30vol.% Al2O3 MMC using stir casting and investigated the effect of
Al2O3 content and size on the mechanical properties of the MMC. Kerti and Toptan
(2008) used stir casting to produce Al/B4C MMC and studied the effect of
wettability agent K2TiF6 on microstructural evolution of the MMC. Kalaiselvan et
al (2011) developed AA6061/B4C MMC using stir casting and assessed the effect of
B4C content on microstructural evolution and mechanical properties of the MMC.
15
Sudarshan and Surappa (2008) synthesized A356/12vol.% fly ash using stir casting
and evaluated the damping capacity of the MMC. Gopalakrishnan and Murugan
(2009) prepared AA6061/TiC MMC using modified stir casting method and studied
the effect of TiC content on microstructure and mechanical properties of the MMC.
2.2.4 Compo Casting
The principle of compo casting is identical to stir casting. The only
difference is that the temperature is maintained such that the matrix material will
not fully melt but remains in semi solid state. The semi solid matrix material is
called as slurry. Hence, this process is also known as slurry casting. Adding
ceramic particles to the slurry improves wettability and provides more uniform
distribution.
Rajan et al (2007) fabricated Al/fly ash MMC using compo casting and
compared the properties with similar MMC fabricated using stir casting. Vencl et al
(2010) developed A356 reinforced with Al2O3, SiC and graphite particles using
compo casting and studied the effect of heat treatment on microstructural evolution
of the MMC. Amirkhanlou and Niroumand (2010) prepared A356/SiC MMC using
compo casting and observed enhanced properties compared to similar MMC
fabricated using stir casting.
2.2.5 Squeeze Casting
Porous preforms of reinforcement material are infiltrated by molten metal
under pressure to produce MMCs. Figure 2.4 shows the typical processing steps of
squeeze casting of composites which are described as follows. The molten matrix
material is poured into a mold. The ceramic preform shaped to match the contours
of the mold is infiltrated by the molten metal under pressure. A hydraulically
activated ram applies a low controlled pressure to the molten metal to attain
infiltration of the preform without damaging it. Infiltration may or may not be
vacuum assisted. Once infiltration is complete, a high pressure is applied to
eliminate the shrinkage porosity that can occur when the liquid metal contracts as it
16
transforms into the solid state. This complete consolidation or absence of porosity
provides the squeeze cast MMC with excellent mechanical properties. Various
reinforcement materials including carbon, graphite, and ceramics, such as oxides,
carbides, or nitrides are used. The reinforcement may be in the forms of continuous
fiber, discontinuous fiber, and particulate. Matrix materials used are aluminum,
magnesium, copper, and silver. The volume fraction of reinforcement in the metal
matrix composites varies from 10 to 70 depending on the particular application for
the material.
Figure 2.4 Squeeze Casting Process (Lii et al 2002)
Lii et al (2002) fabricated Al/AlN MMC using squeeze casting and
investigated the effect of applied pressure on the content of AlN of the MMC. The
content of AlN particles increased as applied pressure was increased. Zhang et al
(2003) prepared Al/50vol.% AlN MMC using squeeze casting and evaluated
mechanical and thermal properties of the MMC. Celaya et al (2007) developed
Al/SiC MMC using squeeze casting and estimated the impact strength of the MMC.
17
2.2.6 Spray Deposition
Spray deposition techniques fall into two distinct classes, depending on
whether the droplet stream is produced from a molten bath (Osprey process) or by
continuous feeding of cold metal into a zone of rapid heat injection (thermal spray
process). Figure 2.5 shows a typical spray forming setup. The matrix metal is
melted in a crucible kept in a furnace. The molten metal is passed through a
downward pipe. A predetermined quantity of ceramic particles is injected into the
downstream of molten metal. The transfer mechanism rotates the downstream pipe
which creates a stirring and mixing action. Then, the composite melt will be poured
into a die or deposited on a metal substrate. The solidified composite can be
subjected to other secondary processes. The spray process has been extensively
explored for the production of MMCs by injecting ceramic particles into the spray.
MMCs produced in this way often exhibit inhomogeneous distribution of ceramic
particles. Porosity in the as sprayed state is typically about 5–10% (Kaczmar et al
2000). MMCs processed by spray deposition technique are relatively inexpensive
with the cost that is usually intermediate between stir cast and powder metallurgy
processes.
Figure 2.5 Spray Forming Process (Kaczmar et al 2000)
18
Zhitao and Zhenhua (2001) prepared AA6066/SiC MMC using spray
forming process and analyzed the influence of process parameters on the
distribution of SiC particles. Zambon et al (2003) fabricated A357/SiC MMC using
spray forming process and compared the mechanical properties of the MMC with
those of the unreinforced alloy. Srivastava and Ojha (2005) synthesized Al/SiC
MMC using spray forming process with variation in particle flow rate, size of
reinforcement particles and their volume fraction and studied the microstructure of
the MMC.
2.3 IN-SITU FABRICATION OF MMCs
Liquid method of processing is effective owing to its simplicity, easy of
adaption, and applicability to large quantity fabrication. Liquid method of
processing involves either adding ceramic particles externally to the molten metal or
synthesizing in the melt itself. The former is known as ex-situ fabrication (stir
casting, squeeze casting and spray deposition) as discussed earlier while the later is
called as in-situ fabrication.
In-situ fabrication involves synthesizing the reinforcements by chemical
reactions between elements or between elements and compounds. Figure 2.6 shows
the in-situ fabrication of MMCs schematically. The matrix alloy is melted in a
furnace. The measured quantity of reaction elements/compounds is incorporated
into molten matrix material to synthesize reinforcements. Then, stirring is continued
for some time to disperse reinforcements uniformly into the matrix. Stirring should
not be rigorous as done in stir casting which will lead to entrapment of floating
reaction products into the matrix alloy. After removing the reaction products i.e
slag, the composite melt is poured into moulds. The in-situ reaction is exothermic in
nature. The rise in temperature depends on the nature of elements/compounds added
into the melt. The set temperature of the furnace should be enough to initiate and
sustain the reaction.
19
Figure 2.6 Schematic of In-Situ Fabrication of MMCs (Tjong and Ma 2000)
In-situ fabrication produces fine size of ceramic particles. The size of the in-
situ formed particles is influenced by synthesis temperature, holding time, reaction
rate and cooling rate (Tjong and Ma 2000). The maximum percentage of
reinforcement is limited by the amount of slag formed and consumption of matrix
material during in-situ reaction. The melt becomes highly viscous when particulate
content is increased beyond a critical value. As a result sound castings cannot be
obtained (Kumar et al 2008).
The surface of the in-situ formed particle tends to be free of contamination
which improves the interfacial bonding strength. Uniform distribution of fine size of
particles is effortlessly achieved without the need for addition of wetting agent. In-
situ formed particles exhibit higher degree of thermodynamic stability which
enables to avoid the formation of undesirable phases. More over in-situ fabrication
is a single step economical process (Hoseini and Meratian 2005, Ramesh et al
2010). This fabrication method is employed in the present research work to exploit
those advantages.
20
Table 2.1 presents a list of the reported research works on in-situ fabrication
of MMCs over the last decade. There has been a constant interest to develop MMCs
using in-situ reaction of different elements. Several MMCs have been successfully
fabricated by this method. Different ceramic reinforcements such as TiC, Al2O3,
TiB2 and ZrB2 were synthesized. The percentage reinforcement was limited to 10 in
most of the reported works.
Synthesizing temperature or reaction temperature plays a crucial role in in-
situ fabrication. Insufficient synthesizing temperature will lead to incomplete
reaction which will introduce brittle intermetallic compounds into the matrix. Birol
(2008) observed that the in-situ reaction between graphite and K2TiF6 was strongly
influenced by the synthesizing temperature. The formation of intermetallic
compound Al3Ti was suppressed or aggravated depending on the synthesizing
temperature. Zhao et al (2007) reported that the synthesizing temperature influenced
the morphology of the intermetallic compound Al3Zr. Several investigators used
8500C - 900
0C as synthesizing temperature to form TiB2, ZrB2 and Al2O3 particles
(Natarajan et al 2009, Wang et al 2010, Kumar et al 2010a). But in-situ forming of
TiC particles requires a synthesizing temperature more than 1000OC (Tyagi 2005,
Shyu and Ho 2006, Birol 2008, Liang et al 2010, Kumar et al 2010c).
The ratio of elements/compounds added is another significant factor which
governs the formation of particles. The possible reactions among the elements
added should be explored prior to investigation. The stoichiometric ratio of
elements/compounds required to make the reaction complete should be calculated.
It was evident from the literature survey that the stoichiometric ratio of elements to
be added was not calculated clearly in some investigations while this data was not
presented in many works. The elements were added on trial and error basis to form
particles which caused the formation of brittle intermetallic compounds due to
incomplete reaction (Hamid et al 2005, Sheibani and Najafabadi 2007, Birol 2008,
Liang et al 2010, Ramesh et al 2010).
21
Table 2.1 List of Research Work of In-Situ Fabrication of MMCs
S.No Reference Type of In-situ
MMC
Amount of
Reinforcement
Reaction Elements/
Compounds
1 Han et al 2002 Al-12Si/TiB2 0-7 wt.% K2TiF6 and KBF4
2 Liuzhang et al 2003 Al-12Si/Al2O3 10 wt.% Al2(SO4)3
3 Hoseini and
Meratian 2005
Al/Al2O3 0-5 wt.% CuO2 and glass
powder
4 Tyagi 2005 Al/TiC 0-18 vol.% SiC and Ti
5 Hamid et al 2005 Al/Al2O3 5 wt.% MnO2
6 Zhao et al 2005 Al/TiB2 and ZrB2 --
K2TiF6, K2ZrF6 and
KBF4
7 Shyu and Ho 2006 Al-5.1Cu/TiC 6 vol.% Al–5.1Cu–6.2Ti alloy
and CH4 gas
8 Hamid et al 2006 Al/Al2O3 5 wt.% TiO2
9 Sheibani and
Najafabadi 2007
Al/TiC 10 wt.% Graphite,TiO2 and
Na3AlF6
10 Zhao et al 2007 Al/Al3Zr and
ZrB2
20 wt.% K2ZrF6 andKBF4
11 Mandal et al 2007 Al-4Cu/TiB2 0-10 wt.% K2TiF6 and KBF4
12 Kumar et al 2007 Al-4Cu/TiB2 0-10 wt.% K2TiF6 and KBF4
13 Birol 2008 Al-Ti/TiC 10 wt.% Graphite and K2TiF6
14 Zhang et al 2008 A356/Al3Zr and
ZrB2
0-25 wt.% K2ZrF6 andKBF4
15 Kumar et al 2008 Al-7Si/TiB2 0-10 wt.% K2TiF6 and KBF4
16 Herbert et al 2008 Al-4.5Cu/TiB2 5 wt.% K2TiF6 and KBF4
17 Zhao et al 2008 Al-4Cu /Al2O3,
ZrB2 and Al3Zr
4-16 vol.% Zr(CO3)2 and B2O3
18 Mandal et al 2009 A356/TiB2 0-10 wt.% K2TiF6 and KBF4
19 Natarajan et al 2009 AA6063/TiB2 0-10 wt.% K2TiF6 and KBF4
20 Ji et al 2009 Al-4.5Cu/TiC 15-20 vol.% Graphite and Ti
21 Ramesh et al 2010 AA6063/TiB2 10 wt.% Al–10Ti and Al–3B
22 Kumar et al 2010a AA6351/ZrB2 0-9 wt.% K2ZrF6 andKBF4
23 Wang et al 2010 Al/Al2O3 3-4.5 vol.% Ce2(CO3)3
24 Liang et al 2010 Al-4.5Cu/TiC 10 wt.% Graphite and Ti
25 Kumar et al 2010b Al-4Cu/TiB2 2.5-10 wt.% K2TiF6 and KBF4
26 Christy et al 2010 AA6061/TiB2 12 wt.% K2TiF6 and KBF4
27 Kumar et al 2010c A356/TiC 5 wt.% Graphite and K2TiF6
28 Tijun et al 2010 Al/Al3Ti 0-10 wt.% K2TiF6
22
Intermetallic compounds such as Al3Ti and Al3Zr exhibit needle shape and
brittleness which deteriorate the mechanical behavior of the MMCs (Zhao et al
2005, Tijun et al 2010). Zhang et al (2008) added the salts K2ZrF6 and KBF4 at 1:1
ratio to produce ZrB2 particles. But the reaction was incomplete which introduced
large amount Al3Zr. Kumar et al (2010a) added those salts at 1:2.4 ratio and found
that the fabricated MMC contained ZrB2 particles alone without the presence of
Al3Zr due to complete reaction. An increase in holding time would help the in-situ
reaction to complete. But it caused coarsening of particles and porosity (Tjong and
Ma 2000).
All type of in-situ formed particles displayed good wettability with the
matrix material. The increase in local melt temperature due to exothermic reaction
enhances the wettability of particles (Han et al 2002). Any kind of ceramic particle
can be successfully incorporated into the matrix alloy by in-situ fabrication. Hence,
it is possible to overcome the limitation of stir casting method. The interface of
particles and the matrix was found to be clean. The particles were not surrounded
with reaction products. A pure interface increases the load bearing capacity of the
composite. The in-situ formed particles significantly refined the microstructure of
matrix alloys and distributed uniformly in the matrix. The uniform distribution of
ceramic particles is superior to ex-situ MMCs. The grain refinement and uniform
distribution helps to enhance the mechanical and tribological behavior of MMCs
(Kumar et al 2010a). The tensile strength of in-situ MMCs is increased with an
increase in particulate content as shown in Table 2.2.
Table 2.2 Tensile strength of Typical In-Situ MMCs
S.No Reference Type of In-situ
MMC
UTS of Matrix
Alloy (MPa)
UTS of MMC
(MPa)
1 Han et al 2002 Al-12Si/TiB2 208 275
2 Hoseini and
Meratian 2005
Al/Al2O3 180 200
3 Kumar et al 2008 Al-7Si/TiB2 146 209
4 Ramesh et al 2010 AA6063/TiB2 95 145
23
Table 2.2 Continued
S.No Reference Type of In-situ
MMC
UTS of Matrix
Alloy (MPa)
UTS of MMC
(MPa)
5 Liang et al 2010 Al-4.5Cu/TiC 252 411
6 Christy et al 2010 AA6061/TiB2 135 174
2.4 DRY SLIDING WEAR BEHAVIOR OF IN-SITU MMCs
MMCs are replacing monolithic alloys in many applications where
components slide each other. The sliding action results in wear of the components.
Therefore testing the wear rate of the MMC is essential before converting into an
application. Pin-on disc wear apparatus has been extensively used by researchers
across the globe to test the wear rate of the MMCs. Figure 2.7 shows the typical
pin-on-disc test setup. The pin which is made of the MMC to be tested is slid
against the hardened steel disc. When no lubricant is used in the test it is known as
dry sliding wear. The factors which influence the wear rate of the MMC are sliding
velocity (V), sliding distance (D), normal load (F), external temperature and type,
size, shape and content of ceramic particles (Sannino and Rack 1995).
Figure 2.7 Schematic Diagram of Pin-On-Disc Test Setup (Rao et al 2009)
24
Table 2.3 shows a list of dry sliding wear testing parameters used for in-situ
MMCs. The effect of percentage of ceramic particles and normal load has been
studied in those works. In-situ formed ceramic particles (TiC, TiB2 and ZrB2)
improved the wear resistance of the composite at all volume fraction. This was
attributed to the good interfacial bonding between the matrix alloy and the ceramic
particles. The ceramic particles refined the grains of matrix alloy which also
contributed to the improvement of wear resistance. Herbert et al (2008) and Kumar
et al (2010a) respectively reported that subjecting the cast MMC to rolling and heat
treatment would further improve the wear resistance. The dry sliding wear behavior
of in-situ MMCs was observed to be non linear. The increase in normal load
increased the wear rate of in-situ MMCs similar to the behavior of MMCs
fabricated from other processes.
Table 2.3 List of Dry Sliding Wear Testing Parameters used for In-situ MMCs
S.NO Reference In-situ MMC Wear Parameters
V (m/s) D (km) F (N) 1 Tyagi 2005 Al/TiC 1 2.1 10-25 2 Mandal et al 2007 Al-4Cu/TiB2 1 1.8 20-80 3 Zhang et al 2008 A356/Al3Zr and ZrB2 0.42 0.73-3.0 20-100
4 Kumar et al 2008 Al-7Si/TiB2 1 0.8 40-120 5 Herbert et al 2008 Al-4.5Cu/TiB2 1 1.8 20-80 6 Zhao et al 2008 Al-4Cu /Al2O3, ZrB2
and Al3Zr 0.42 0.5-3.0 100
7 Mandal et al 2009 A356/TiB2 1 1.8 20-80 8 Kumar et al 2010a AA6351/ZrB2 1 1.2 10
2.5 CORROSION BEHAVIOR OF MMCs
The MMCs exhibit superior mechanical and tribological behavior compared
to monolithic alloys. But MMCs are not known for their corrosion behavior. One of
the main obstacles to the use of MMCs is the influence of reinforcement on
corrosion resistance which is particularly important in aluminum alloy based
composites. The incorporation of ceramic particles is generally detrimental to the
25
corrosion resistance of MMCs. When aluminum alloys are exposed to atmosphere
or other corrosive media a protective oxide film is formed on the surface which
imparts corrosion resistance. This process is known as passivation. The addition of
ceramic particles leads to discontinuities and flaws in the oxide film increasing the
number of sites where corrosion can be initiated and rendering the composite
susceptible to severe attack. Pitting attack is the major form of corrosion in MMCs.
When MMCs are subjected to NaCl solution, more pits are formed in composites
than that of unreinforced alloys. The matrix and ceramic particle interface is prone
to corrosion (Pardo et al 2005). Several investigators observed an increase in
corrosion rate of MMCs when percentage of reinforcement particles was increased.
Ceramic particles are found to reduce the corrosion resistance of MMCs
(Dobrzanski et al 2005, Kiourtsidis and Skolianos 2007, Tijun et al 2010).
2.6 FRICTION STIR WELDING (FSW) PROCESS
A very novel and potentially revolutionary welding method was conceived at
The Welding Institute, United Kingdom in 1991. The process was named as
Friction Stir Welding. FSW is in consistent with the more conventional methods of
friction welding which have been practiced since the early 1950s.
Figures 2.8 and 2.9 show the schematic of FSW process and the sequence of
FSW respectively which are explained as follows. A cylindrical, shouldered tool
with a profiled probe is rotated and slowly plunged into the joint line between two
pieces of sheet or plate material, which are butted together. The parts have to be
clamped onto a backing bar in a manner that prevents the abutting joint faces from
being forced apart. Frictional heat is generated between the wear resistant welding
tool and the material of the work pieces. Sufficient dwell time is allowed in order to
generate frictional heat. This heat causes the material to soften without reaching the
melting point and allows traversing of the tool along the weld line. The plasticized
material is transferred from the leading edge of the tool to the trailing edge of the
tool profile and is forged by the intimate contact of the tool shoulder and the pin
26
profile. This produces a solid phase bond between the two pieces (Sanderson et al
2000).
Figure 2.8 Schematic Diagram of Friction Stir Welding (Mishra and Ma 2005)
Figure 2.9 Sequence of Friction Stir Welding (Adamowski and Szkodo 2007)
27
The side where the direction of rotation is the same as that of welding is
called the advancing side (AS), with the other side designated as being the
retreating side (TS). The material movement around the pin can be complex due to
various geometrical features of the tool. During FSW process, the material
undergoes intense plastic deformation resulting in generation of fine and equiaxed
recrystallized grains. The fine microstructure in friction stir welds produces good
mechanical properties. FSW is considered to be the most significant development in
metal joining in a decade and is a green technology due to its energy efficiency,
environment friendliness, and versatility (Mishra and Ma 2005).
In contrast to the traditional friction welding, which is usually performed on
small axisymmetric parts that can be rotated and pushed against each other to form
a joint, friction stir welding can be applied to various types of joints like butt joints,
lap joints, T joints, and fillet joints. Though FSW was primarily developed to join
aluminum alloys intense research has been carried out to join other alloys such as
magnesium, copper, brass, steel, nickel and titanium (Nandan et al 2008).
Figure 2.10 shows the Fish-bone diagram depicting the friction stir welding
factors which influence the joint properties as listed below.